Isambard Kingdom Brunel.
Box Hill, near Corsham in Wiltshire, was the highest point of Brunel’s proposed Great Western Main Line from London to Bristol. The route would consist of mainly level ground and gentle gradients, but Box Hill would require a tunnel that contemporary engineers had never seen before. George Stephenson thought that passengers would be terrified by such a long, deep tunnel.
Although the length was ambitious, the tunnel was also built at a 1 in 100 gradient. It was unheard of at the time that a tunnel would be so steep. The tunnel would also have to be cut through a thick layer of Great Oolite (or Bath Stone), a limestone used as a building material. In September 1836, six shafts were sunk into the hill. They were 28 feet in diameter and the deepest was 290 feet.
Original plan drawing, showing the gradient required in constructing the tunnel.
Tunnelling work began in December 1838; the primary contractor was Mr George Burge of Herne Bay. His company began working from the western end and would construct 75% of the tunnel length. Working from the east were two local contractors, Lewis and Brewer. The work was divided into six sections, accessible by the shafts already cut. Steam-powered winches were used to get men and materials in and out of the shafts.
A tonne of candles were used per week to provide lighting. The eastern section, where the rock was thickest, was excavated by blasting. This section was unlined. The western section was dug with pick and shovel and lined with millions of bricks made nearby in Chippenham. Canal tunnel building techniques were used on this part.
During the winter months, the partly constructed tunnel would fill with water, with a section flooded one occasion when the pump was overcome. Around a hundred navvies were killed during construction, mainly due to accidents during blasting and water ingress.
Work progressed slower than Brunel expected and the tunnel was the last part of the GWML to be completed. In August 1840 Brunel increased the workforce to get the project back on schedule. The tunnel was completed in April 1841 and was 30 feet wide; wide enough to accommodate two broad gauge lines.
The grand arch over the western portal.
The first train ran through the tunnel in June 1841, when a special train ran the entire length of the GWML from London Paddington to Bristol Temple Meads for the first time. The western portal was visible from the Bath-London road (now the A4), so Brunel designed a grand classical arch to show off his architectural triumph. At 1.83 miles, Box Tunnel was the longest railway tunnel in the world.
The tunnel is completely straight, which has added fuel to the legend that the sun shines through the tunnel on Brunel’s birthday, 9th April. Mathematicians and engineers suggest that the phenomenon could happen a few days earlier than the ninth, which means that Brunel may not have taken atmospheric refraction into account. In April 2017, the line was closed so a team from GWR tested the theory. They found that the sun did not shine fully through the tunnel on Brunel’s birthday, but the sunrise was well aligned with the eastern portal.
The sun aligning with the eastern entrance on 9th April, 2017. Photo:GWR.
In the summer of 2015, the line was closed for six weeks for electrification to be completed. The track bed was lowered, and overhead catenary was installed. Box tunnel remains an important landmark on the Great Western Mainline.
